Well the biggest disadvantage of not having a CMS of any kind would be if you want to make changes to the content of a webpage you would have to do it in the HTML page. Then you would have to upload it to the remote server. Whereas, a CMS allows you to logon remotely to the website and make changes on the fly.

In general, not using a CMS system has the advantage of more flexibility, if you are competent enough to implement it and maintain it correctly. With a CMS however it is much more convenient, as Strider64 said, you can log in from anywhere with an internet connection and easily edit or add new content. If you are less technically competent, I would recommend having a CMS system.

There are some good ones, as you mentioned, Drupal, Wordpress etc. I personally use EE(ExpressionEngine), purely because I got used to it and it allows me to have a greater degree fo flexibility, you can check out more about it here.

You should read about the different CMS systems available, and the advantages of each one, and then decide what's best for you.

I've seen clients spend big money on a 100% custom CMS tailored to some aspect of their business. Less than 12 months later nobody at the company is using it. 6 months after that somebody at the company is calling me telling me to update their site as if they forgot they had a CMS. This kept happening. We gave them a free tutorial session showing how to use their CMS. 3 months after that somebody at the company is calling me telling me to update their site as if they forgot they had a CMS again. So we give up and charge $100 an hour to update their site for them using the CMS we built for them.

Since the CMS was totally custom it wasn't the best code. There were security vulnerabilities. The quickest and easiest fix for that was to just delete the CMS. Nobody was using it anyway. Such a huge wast of our time and their money... and our money too! The person telling us how they wanted the CMS to function was an indecisive, inconsistent asshole. We definitley lost money on that project.

It would have been better for everyone if we never built them a CMS and they sent small, sane, clearly written e-mails whenever they wanted 1 thing changed instead of an e-mail every other month containing scores of changes, 60% of which they could have done themselves with the CMS, 20% of which were not possible with the CMS, and 20% of which were not possible in the reality of this 3 dimensional universe we live in.